This invention relates to an apparatus for measuring the concentration of oxygen in a fluid, the apparatus being of the type having a layer of an oxygen ion conductive solid electrolyte.
It has been popularized to detect the oxygen content in the exhaust gas from an automotive internal combustion engine as the basis for the control of the air-to-fuel ratio of a combustible mixture fed to the engine by the use of an oxygen sensor that produces an electrical signal representing the detected oxygen content. For such purpose, usually use is made of an oxygen sensor that operates on the principle of a concentration cell and has a layer of a solid electrolyte having a high oxygen ion mobility, e.g. zirconia stabilized with calcia, and two porous and electronically conductive electrode layers of platinum, by way of example, formed respectively on both sides of the solid electrolyte layer. When one side of the solid electrolyte layer in this sensor is exposed (through one of the electrode layers) to a sample gas having an unknown oxygen partial pressure Po and the other side to a reference gas such as air whose oxygen partial pressure Po' is known and there is a difference between the oxygen partial pressures Po and Po', the sensor develops an electromotive force E across the two electrode layers. This electromotive force E is related to the oxygen partial pressures Po and Po' by the Nernst equation: ##EQU1## where R is the gas constant, T represents the absolute temperature, and F is the Faraday constant. Accordingly the oxygen content in the sample gas can be detected by measuring the electromotive force E with a potentiometer connected to the two electrode layers of the sensor while the total pressure of the sample gas is kept constant.
Conventional oxygen sensors of this type are designed in various ways. As the most popular example, the solid electrolyte layer takes the form of a tube closed at one end so that air used as a reference gas can be introduced into the interior of the tube while the outside of the tube is exposed to a sample gas. The solid electrolyte tube is produced by a sintering technique and has a wall thickness large enough to serve as a structurally basic member of the sensor. However, this oxygen sensor has disadvantages such as unsatisfactory temperature dependence of its output characteristic originating from a large heat capacity and high electrical resistance of the voluminous solid electrolyte tube, insufficiency of the mechanical strength and heat resistance of the solid electrolyte tube and considerably high production cost. When the size of the solid electrolyte tube is reduced to diminish these disadvantages, it becomes necessary to continuously supply a fresh reference gas to the interior of the solid electrolyte tube during measurement, and there arises a problem that the oxygen partial pressure in the electrode layer on the inside of the solid electrolyte tube tends to deviate from the real oxygen partial pressure of the reference gas since a diffusion layer is formed at the surface of this electrode layer by an electrode reaction of the reference gas, resulting in the fact that the oxygen content in the sample gas can no longer be measured accurately. To succeed in accurate measurement of the oxygen content in a sample gas, the solid electrolyte tube in this type of oxygen sensors must provide an interior space large enough in volume to allow continuous replenishment of the reference gas in order that an oxygen concentration gradient created in the vicinity of the inner electrode layer by diffusion of the reference gas is kept as small as possible. It is impractical, therefore, to employ a very small-sized solid electrolyte tube in this type of oxygen sensors.
As a variation of the above described type of oxygen sensors, it has been proposed to confine a volume of reference gas in a closed space in which is disposed one of the electrodes of the concentration cell. However, another disadvantage arises by this modification. During use of this sensor, the oxygen partial pressure of the reference gas confined in the closed space varies as the oxygen in the reference gas is consumed gradually by electrolysis. When the reference oxygen partial pressure is lowered beyond a certain level, it becomes necessary to replenish the closed space with oxygen gas by suspending the measurement and applying a voltage to the solid electrolyte of the sensor with its outer electrode layer (measurement electrode) exposed to another reference gas such as atmospheric air until the sensor resumes the ability of generating a standard level of electromotive force. This sensor, therefore, is unsuitable for continuous and longterm measurement of oxygen concentration in an exterior atmosphere. If it is wished to use this oxygen sensor continuously for a long period of time, the volume of the closed space must be large enough to confine such a large quantity of reference gas that the oxygen partial pressure in the closed space does not exhibit a significant change during use of the sensor. When this requirement is met, the sensor necessarily becomes very large in total size. As an additional disadvantage, this oxygen sensor is rather inferior in its accuracy of measurement because the reference gas confined in the closed space scarcely flows in this space and hence a considerably thick layer of stagnant reference gas is produced along the surface of the inner (reference) electrode layer. Besides, this oxygen sensor is complicated in structure, inconvenient to mass-production and unsatisfactory in mechanical strength and heat resistance.
The disadvantages of the above described oxygen sensors originate primarily from the use of a gas as the source of a reference oxygen partial pressure. Accordingly it has been proposed to utilize a metal-metal oxide system, which exhibits a sort of buffer function derived from a thermodynamic equilibrium, as the source of reference oxygen partial pressure for the concentration cell type oxygen sensor. Such a metal-metal oxide system, e.g. Ni--NiO system, can be formed into a porous layer which is included in the oxygen sensor. Then, a small-sized oxygen sensor can be produced by the employment of thin film and/or thick film techniques. For example, an oxygen sensor of this type comprises a substrate such as of an alumina plate, a reference electrode film deposited on the substrate, a metal-metal oxide film on this electrode, a solid electrolyte film on the metal-metal oxide film and a measurement electrode film on the outside of the solid electrolyte film. This oxygen sensor has many advantages but also has the following disadvantages. When this oxygen sensor is used in a high temperature atmosphere having a relatively high oxygen partial pressure, the reference oxygen partial pressure developed by the metal-metal oxide reference system tends to vary, with the result that the output of the sensor varies with the lapse of time, because of the oxidation of the metal in contact with the solid electrolyte and/or the influence of the exterior gas atmosphere on the reference system. This means that this oxygen sensor does not have a satisfactorily long service life. Besides, this oxygen sensor suffers a considerably high production cost since it is not easy to form a thin layer of a metal-metal oxide system (e.g. by sputtering, electro-deposition or vacuum evaporation) with the maintenance of the most desirable mole ratio (about 1:1) of the metal to the metal oxide.